Why a local Overlay?
If you only want to put one ebuild into your Portage tree and do not want to add a complete Overlay (like Sabayon, Gnome or KDE), this is your way. By using a local Overlay you can keep your stable Gentoo clean and have some unstable Packages from Overlays as well.

Now go to http://gpo.zugaina.org/ and search for the ebuild you want to add to your Portage tree. Download something like "foo.ebuild". The ending .ebuild is important. For foo you now have to create the folders "category" and "foo" inside "category"

The foo version I used as an example is a stable one. If you add unstable ebuilds to your Portage tree, you have to unmask them in /etc/portage/package.unmask.
If your ebuild has dependencies that can not be resolved with the offical Portage tree, you have to add them to your local overlay as well.

Last edited by FelixPetzold on Mon May 10, 2010 5:49 pm; edited 3 times in total

Yes, after eix-update (if you have set PORTDIR_OVERLAY appropriately). Concerning speed of eix-update, you might want to have a look at the description of the CACHE_METHOD and related variables in the eix manpage.